Now for what I always wanted to try, a tube Franklin VFO! Even better, using sub-miniature tubes. The Franklin oscillator topology uses two tubes, and a pair of very small capacitors to couple the tuned circuit to the amplifier input and feedback. The capacitors are supposed to be as small as possible but still allow reliable oscillation. This isolates any nasty effects such as loading and variations in capacitance in the tube from the tuned circuit, and keeps the tuned circuit Q high. Theory says that this produces a very very stable oscillator - we shall see.

Not much is written about Franklin VFO's, even less about tube Franklin oscillators, and nothing at all about sub-mini tube variants. Some useful links:

The circuit I ended up with is below. Note that in a Franklin oscillator the cathodes are grounded. This is really perfect for battery valves, which are directly heated i.e. the filament IS the cathode! No need for RF chokes etc. I used my 12V regulated power supply and some series resistors, to get 1.25V 20mA for each of the tubes.

Here's a photo of the Franklin VFO, and one of the very nice 'scope waveform.

I added a measurement of the output amplitude to the previous chart (from the single-6088 tube oscillator). The Franklin VFO is the yellow line. The circuit would not oscillate below a supply of 41.5V, and the output amplitude is less than the single-6088 oscillator. Nevertheless, the Franklin is supposed to be STABLE, not high output. So let's not dwell on issues of low output amplitude which can always be restored later using a buffer amplifier if required - and anyway 1.5V p-p is quite usable and respectable.

The following chart shows output amplitude vs frequency of this Franklin oscillator.

Before making measurements of the frequency drift, I'll put this oscillator in a box. That's next - first, a word about the tuning capacitor, read on...